Parallel pathways in macaque monkey striate cortex: anatomically defined columns in layer III.

Visual information reaching striate cortex comes from parallel pathways, and the information is organized, or processed, by the layers and columns of striate cortex. To better understand how this is accomplished anatomically, we asked whether parallel pathways originating in the lateral geniculate nucleus (LGN), and terminating separately in layer IV, remain separate in layer III of macaque monkeys. Layer III is of interest since it may play a special role in color and form vision but not in analysis of visual motion. The chief finding was that cells in "blobs" of layer III that stain densely for cytochrome oxidase receive indirect input, via layer IVC, from both LGN magnocellular (M) and parvocellular (P) cells. This is important because the P and M pathways may represent color/form and motion-processing channels, respectively. Interblob cells receive indirect input, via layers IVC and IVA, from the LGN P cells. Also, as suggested by others, our data demonstrate that layer III can be subdivided. The bottom tier, layer IIIB, receives direct projections from all cortical layers. Output from layer IIIB appears to remain intrinsic to striate cortex. In contrast, the top tier, layer IIIA, receives projections from layer IIIB as well as from layers IVA, IVB (blobs only), and V, but it receives no direct projections from LGN recipient layers IVC and VI. Unlike layer IIIB, the output of layer IIIA reaches extrastriate areas. Thus, impulses arriving from parallel LGN pathways may be recombined through serial stages in striate cortex to produce a set of parallel pathways that are qualitatively different from the original LGN set.